Cross-linking diacetylenes

Figure 58 The carboranylenesiloxane (95) containing the cross-linking diacetylene group. (Adapted from ref. 125.)... 

Another example of cross-linked monolayers is that of polymerized diacetylenic thiols. Well-packed monolayers of such thiols, HS(CH2)ioC=CC=C(CH2)ioCOOH, can be polymerized in situ by UV irradiation. The polymeric structures obtained are extremely robust and durable compared to similar monomeric layers. They can be repeatedly (more than 30 times) scanned between potentials —0.5 to —1.4 V in 0.5 M KOH without any appreciable signs of desorption. Heating of the films to 200 °C for 1 h showed only ca 15% decrease in the IR intensities of the CH2 peaks, and exposure to 1 M KOH at 100 °C did not affect the integrity of the monolayer306. 

The similar main chain structures with conjugated diacetylene units (34, 35) can be prepared (equations 32 and 33) by polyaddition or polycondensation using components containing the conjugated diyne units . The processable polymers become insoluble on photoirradiation or by thermal annealing via intermolecular cross-linking. 

Hyper-Cross-linked Organic Solids—Preparation from Poly(aromatic)diacetylenes and Prellmlnaiy Measurements of Their Young Modulus, Hardness and Thermal Stability... 

FIGURE 42.2 UV-induced cross-linking in diacetylene LB films. Percent polymer conversion plot is based upon the 288/ Ag ratio (intensity ratio between a daughter ion representative of the analyte molecule and the silver-related signal) versus time of UV exposure. Figure from Reference [28]. 

Because of the versatility of the polyurethane system it is possible to introduce comonomers which can affect the physical properties of the derived polymers. For example, photo cross-linkable polyurethanes are formulated using 2,5-dimethoxy-2,4 -diisocyanato stilbene as a monomer (76). Comonomers, having an azoaromatic chromophore, are used in optical bleaching applications (77), or in the formation of photorefractive polymers (78). The latter random poljnners have second-order nonlinear optical (NLO) properties. Linear poljnners are also obtained from HDI/PTMG and diacetylenic diols. These polymers can be cross-linked through the acetylenic linkages producing a network polymer with properties similar to poly(diacetylenes) (79). 

If dispersions are vigourously probe sonicated until only small unilamellar liposomes remain, no polymerisation occurs on irradiation. ) Most diacetylene polymers are highly crystalline an insoluble in all solvents. However, with one exception, phospholipi polymers will dissolve in chloroform or chloroform-alcohol mixture The exception is the polymer formed when same chain lipid is subjected to prolonged irradiation. These polymers are probably cross-linked. A polymer chain cross-linking reaction is only possible when the monomer contains more than one diacetylene group. 5)... 

Due to the high cytotoxicity of Cu(I) ion and the difficulty in completely removing it after the CuAAC reaction, it becomes highly desirable to promote the [2-1-3] cycloaddition between the acetylene and the azide under Cu ion-free conditions. In this coimection, Kiser reported the in situ preparation of cross-linked hydrogels by using a Cu(I)-free Huisgen cycloaddition between an azide-functionalized methacrylate polymer 37 and an activated diacetylene 38 (Scheme 11) [51]. However, even with 38 as the activated dipolarophile, the rate of cross-linking was still too slow (10% completion in 49 h at 24°C) to be useful in most in situ applications. 

An interesting class of metal-induced cross-linking gel was reported by Hecht [68]. A poly(triazole-pyridine) 68 was prepared by AABB CuAAC copolymerization of a pyridyl diacetylene 69 and a chiral aryl diazide 70 (Scheme 20). Due to the repulsive interactions between the nitrogen lone-pair electrons on the triazole and pyridine units [69], the polymer tended to adopt a helical conformation in the absence of metal ions. On the other hand, metallogels 71 were formed when Zn (II), Fe(II), or Eu(III) ion was added to a solution of the polymer in acetonitrile. It was believed that the 2,6-bis(triazolyl)pyridine ligands chelated to the metal ion to form a hexacoordinate metal complex, which then led to a metal-promoted cross-linked 3D network that entrapped acetonitrile solvents. 

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